Transpiration

All green plants synthesize their food through the process of photosynthesis and water is an essential component required by the leaves for photosynthesis. Xylems carry water from the roots to the other parts of the plant. Leaves play a vital role in this biological process and stem transports the prepared food to different parts of the plant. Root system plays a significant role in the plant’s survival as they help in transferring water and other minerals. Hence, various parts of plants help in carrying out daily life processes.

Like all living organism, plants too require an excretory system to discharge excess waste from their body and it is the crucial life process as photosynthesis and transportation. But excess water is excreted out from the plants by a mechanism called transpiration.

The excess water in the plant cell is excreted out by a mechanism called transpiration.

Transpiration

Transpiration is a biological process in which water is lost in the form of water vapor from the aerial parts of the plants. This process occurs mainly through the stomata where the exchange of gases (oxygen and carbon dioxide) occurs. The cellular factors that affect transpiration are the number and distribution of stomata, the number of open stomata, the water status of the plant, canopy structure, etc. The environmental factors that affect transpiration are- temperature, light, humidity and wind speed. When water evaporates through the leaves, a pull is created and through the xylem, water moves to the leaves. The ascent of sap that is driven by transpiration depends on the following properties of water:

  • Cohesion – This is the mutual attraction between molecules of water.
  • Adhesion – The attraction of water molecules towards polar surfaces.
  • Surface tension – The molecules of water are more attracted to each other in the liquid phase than in the gas phase.

 Opening and Closing of Stomata

Opening and Closing of Stomata

Stomata consist of a pair of guard cells with an aperture in between. It remains open during the daytime and is closed at night. The reason for the opening and closing of this structure is the turgidity of guard cells. The interior wall of the guard cells present towards the aperture is dense and flexible. The stomata open when the turgidity of the guard cells increases. The exterior walls bulge out and the interior walls form a crescent shape. The orientation of the microfibrils in the guard cells also plays an important role in the opening of the stoma. The radial orientation of the microfibrils makes it easier for the stoma to open. The stomata close when the turgidity of the guard cells decreases due to the water loss and the interior walls form a crescent shape retrieve their original shape.

In dicots, the lower side of leaves have more stomata while in monocots, both the sides have an equal number of stomata.

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Practise This Question

Water evaporates through tiny openings present in the leaves known as: